An LCD (Liquid Crystal Display) forms an electric field in a liquid crystal layer to change a pattern of polarized light passing through it such that it may use the phenomenon that light selectively passes through it.
At an initial stage, LCD devices are generally applied to a small single-color display of an electronic calculator or a clock. However, along with the progress of LCD techniques, LCD devices are at the present used for large color displays due to the development of AM-TFT technology.
In addition, consumers of LCD devices demand not only excellent quality of display that gives clear pictures with a high contrast ratio, high brightness, wide range of view, and prevention of leakage of light, but also harmonization with surroundings even when power is not applied thereto and other additional functions.
To cope with such demands, there was an attempt to provide a LCD device having a mirror surface to act as a mirror when power is intercepted. That is to say, a reflector film coated with a thin metal layer that ensures transmission of light is attached to a surface of a LCD device to form a mirror surface. However, the metal layer included in the reflector film shows a seriously lowered ratio of transmitted light, namely low transmittance, rather than the case not including the metal layer, though it allows transmission of light. As a result, brightness of images delivered to observers from a backlight is not sufficient, so a display screen is not well visible when it is observed at a bright place.
In other words, in case of using a conventional reflector film, the LCD device may give a good mirror effect when power is intercepted, but when power is supplied to the LCD device to display images, the LCD device may not give clear pictures at a bright place due to a reduced contrast caused by inefficient brightness.
Thus, many efforts are continuously made to develop a display device capable of giving a sufficient mirror effect when power is intercepted and also capable of giving a clear display even at a bright place by ensuring sufficient screen brightness when power is supplied thereto.
Meanwhile, a LCD device has very low luminance efficiency since the light reaching eyes of an observer is not greater than 10% of the light emitted from a light source due to the natures of the device.
FIG. 1 is a sectional view schematically showing a conventional LCD device. Referring to FIG. 1, the LCD device includes a light source 10, a backlight unit 40 composed of a light guide plate 20 for transferring a light emitted from the light source 10 and a reflection plate 30 for reflecting the light transferred from the light guide plate toward an observer, a diffusion plate 50 for uniformly diffusing the light reflected by the reflection plate 30 to enhance uniformity, a prism sheet 60 for enhancing brightness of light, a lower polarizer 70 for passing light only in one direction among lights oscillating in various directions, a liquid crystal layer 90 for changing a polarization state of light by changing arrangement of the light passing through the lower polarization plate 70 according to supply of power, and an upper polarizer 80 for passing or intercepting the light transferred from the liquid crystal layer 90 according to the polarization state of light.
In the LCD device, the liquid crystal layer 90 determines transmission and absorption of light using a polarizing phenomenon. Thus, the polarizer intercepts 50% of light emitted upward through the backlight unit 40, and at least ⅔ of remaining light is absorbed in a color filter. In this way, the light is absorbed in other film layers, so just 10% or less of the lights emitted from a light source may reach eyes of an observer.
Thus, in order to realize good quality of display, it is needed to transfer the light emitted from the light source 10 to eyes of an observer at a greatest efficiency. The clearness of display may be represented by a contrast ratio, and a contrast ratio at a bright place (a bright room) may be expressed using the following equation 1.
                              CONTRAST          ⁢                                          ⁢          RATIO          ⁢                                          ⁢                      (                          BRIGHT              ⁢                                                          ⁢              ROOM                        )                          =                                                                                                  BRIGHTNESS                    ⁢                                                                                  ⁢                    OF                    ⁢                                                                                  ⁢                    WHITE                    ⁢                                                                                  ⁢                    LIGHT                                    +                                                                                                      BRIGHTNESS                  ⁢                                                                          ⁢                  OF                  ⁢                                                                          ⁢                  EXTERNAL                  ⁢                                                                          ⁢                  REFLECTED                  ⁢                                                                          ⁢                  LIGHT                                                                                                                                              BRIGHTNESS                    ⁢                                                                                  ⁢                    OF                    ⁢                                                                                  ⁢                    BLACK                    ⁢                                                                                  ⁢                    LIGHT                                    +                                                                                                      BRIGHTNESS                  ⁢                                                                          ⁢                  OF                  ⁢                                                                          ⁢                  EXTERNAL                  ⁢                                                                          ⁢                  REFLECTED                  ⁢                                                                          ⁢                  LIGHT                                                                                        Equation        ⁢                                  ⁢        1            
Seeing the equation 1, it would be understood that, in case a white light has a low brightness, the brightness of an external reflected light gives an increased influence on a contrast ratio, so the contrast ratio may be greatly lowered. Thus, it would reach a conclusion that, if the brightness of the external reflected light is out of control, enhancing brightness of a white light is required for increasing the contrast ratio and thus obtaining a clear quality of display, and also it is required to reduce light leakage such that the brightness of a black light is kept as lower as possible.
Luminance efficiency (namely, a ratio of light reaching eyes of an observer among emitted light) is increased if power supplied to the backlight unit 40 is raised since an amount of light emitted from the backlight unit 40 is increased. However, in a mobile display using a battery (e.g., a cellular phone, a notebook, and PDP), a battery may be rapidly discharged if power supplied to the backlight unit is raised to increase luminance efficiency.
In order to solve the problem of luminance efficiency reduction caused by inherent features of a LCD device, namely the problem that the polarizer absorbs and intercepts a significant amount of light to lower luminance efficiency, 3M proposed DBEF (Dual Brightness Enhancement Film). This DBEF is a reflection polarizer having polymer thin films in multi layers. When being applied to a transmissive LCD, the DBEF is positioned between a backlight unit and a LCD panel to substitute or supplement the lower polarizer 70 of FIG. 1. While a conventional absorption polarizer absorbs light not passing through the polarizer to lower luminance efficiency, the DBEF reflects the light not passing through the polarizer into a lower direction such that the reflected light is reflected on the reflection plate 30 (FIG. 1) again and then reaches the DBEF with a changed polarization direction to pass through the DBEF. Thus, even a light with polarization unsuitable for the polarization condition of the DBEF may be utilized with a changed polarization state. In this way, it was reported that efficiency may be increased about 60% rather than a conventional case.
However, the DBEF is expensive since anisotropic polymer films and isotropic polymer films are optically laminated alternately. Also, the DBEF shows deteriorated reflection efficiency since the reflection efficiency is greatly changed depending on a direction of incident light, and a manufacturing process of the DBEF is very complicated. In addition, the DBEF may enhance brightness, but it may not give a minor effect intended by the present invention.